In rotating machines, and particularly in large rotating systems such as industrial turbines or compressors, reliable protection of the system against overspeed or underspeed is required. The term "overspeed" describes the situation which occurs if the rotating parts of a machine or the like have a rotational speed exceeding a predetermined maximum value. The term "underspeed" refers to the converse situation. An uncorrected overspeed or underspeed condition may result in major damage to equipment and danger to human safety. Either overspeed or underspeed condition may result from instantaneous loss of full load or power, and reliable reaction time of the overspeed/underspeed detector is essential. To prevent damage to the monitored machine, the overspeed/underspeed detector generally initiates a shutdown sequence once an unacceptable speed has been monitored over a prescribed interval.
Existing overspeed/underspeed detection systems can be categorized generally as (1) analogue, (2) digital or (3) combined analogue and digital. Generally, such systems consist of a speed transducer, driven by a rotating shaft of the monitored machine, which transducer produces an output signal whose amplitude or frequency is proportional to the machine's angular velocity; a reference signal generating circuit whose output signal has an amplitude or frequency proportional to the maximum or minimum speed limit; and a comparator circuit which compares the output signal of the transducer with the reference signal to produce a third signal indicative of whether or not a machine is running within acceptable limits. Generally, either the frequency of the transducer signal is compared to a reference frequency proportional to the prescribed speed limit, a number of pulses produced by the transducer over a fixed period of time is compared to a reference number, or the transducer pulse train is applied against a reference pulse train of known characteristics in order to produce a third pulse train indicative of the differential. Existing analogue systems are simple in design but have proved to suffer from inaccuracy due to noise and temperature or aging drift. Combined digital/analogue systems generally require complex circuitry and increased cost. Digital systems are often too slow in response time. Where response time has been improved by monitoring the period of the measured signal rather than the frequency, it has been found to be difficult to vary the reference speed limit particularly where an accessory shaft, rather than the main shaft of the machine, is being monitored.
For example, an existing analogue overspeed/underspeed detector is a direct current tachogenerator which generates a signal whose amplitude is proportional to the angular velocity of the shaft whose speed is being measured. This amplitude may then be compared to a reference level to determine under or overspeed. Noise in the circuit, attenuation during transmission, and variations due to temperature and time render this method inaccurate and necessitate complex compensating circuitry. An alternating current tachogenerator may be used to generate a signal whose frequency is proportional to the measured frequency. This frequency-defined signal may be converted to a voltage-defined signal whose amplitude is then compared to a reference signal. Conversely, an amplitude-defined transducer signal may be connected to a frequency-defined comparison signal. These methods suffer from the same problems associated with other analogue methods.
In another existing overspeed detection method, the phase difference between the source frequency and the reference frequency is used to produce a third signal indicative of the deviation in angular velocity from the desired rate. Alternatively, a signal whose pulse width is proportional to the period of the transducer signal may be compared to a reference pulse train whose pulse width is inversely proportional to the reference speed to obtain a third pulse train whose magnitude and polarity indicate the extent of over or underspeed. These methods require overly complex circuitry to generate the relevant pulse trains, which are not readily variable with regard to reference speed. A similar method, suffering from the same drawback, uses a ramp generator which is enabled for a period of time proportional to the time taken for a monitored machine to complete a fixed number of revolutions, producing a sawtooth wave form whose peak amplitude is proportional to such period. The sawtooth wave form is then applied to a comparator whose resulting square wave is analyzed to indicate the differential from the desired speed.
Existing digital overspeed/underspeed detectors utilize the method of counting the number of pulses generated by the speed transducer for a fixed period of time, comparing this count to a reference count representative of the upper or lower speed limit. A problem with this method is that the counting period must be of sufficient length that the margin of error is within acceptable limits. Where the speed sensor measures a slow accessory shaft or the monitored machine itself is low speed, the time period required to accumulate a sufficient count may delay the output of a control signal beyond reasonable limits.
An alternative digital method is to generate a digital count representative of the period of rotation of the machine, and to convert such count to a voltage or frequency which is then compared to the desired level or is used to control the speed. This latter step requires analogue circuitry with the accompanying disadvantages previously described, and the reference speed limit is not readily adjustable.